In an illumination device of a microlithographic projection exposure apparatus, diffractive optical elements (DOEs) are often used particularly at the entrance in order to produce a desired intensity distribution (e.g. dipole or quadrupole distribution) by means of an angular emission characteristic of the DOE that is defined by the respective diffracting surface structure, e.g. in a pupil plane.
When used in ranges of high energy density of the laser beam, the material of the DOE is subjected to very great loading. With the use of a DOE composed of quartz glass, the short-wave laser light leads to so-called compacting, i.e. local density changes and local anisotropies in the material. A further non-deterministic birefringence distribution is finally established also owing to absorption and the associated heat input and the resultant strains of the material, and destroys the polarization state of the laser light, which can then only be avoided by exchanging the DOE.
A destruction of the polarization state of the laser light can also result when the DOE is formed from isotropic crystal material, owing to crystal structural defects, stress-induced and/or intrinsic birefringence.